What is it about cedar trees that makes it difficult for some types of plants to grow near it ? Is it some chemical it gives off ?
Probably just shade and root competition, and maybe also the mulching effect of the old fallen needles. I have seen shade-tolerant plants growing under them OK. Also, cedars are often grown as lawn specimens, where the ground under them is mown; provided there is enough light (with low branches pruned off), grass grows perfectly well under them, as in this pic: http://commons.wikimedia.org/wiki/Image:Morlanwelz_Mariemont_JPG01.jpg
Low-branching conifers generally are often dry and rooty beneath. Some kinds are prone to fungi invading the trunks near the ground and appear to protect themselves by deflecting rain with their dense crowns.
